Which 85mm lens are you referring to, specifically? The upcoming, announced, but not-yet-available 85mm DX model has the best MTF graph of any Nikkor lens ever made. Literally, this upcoming lens has the best theoretical optical performance of any Nikon-made lens. Out of hundreds of designs.
The first-generation 85mm PC Micro~Nikkor, the one with the mechanical plunger, has some of the finest performance when topped way down of any telephoto lens.
The second-generation 85mm Micro~Nikkor, the PC-E version with the E or Electronic diaphragm, is also a stellar performer.
It is doubtful that the 18-105 zoom will be as good for critical uses. Decent, yes, but the competing lenses are all superb, top-grade lenses. I guess it depends on how fine a point you want to put on it.